This boss battle is surprisingly simple, especially if you go about your technique the proper way. The middle sister, Cindy, is actually the one you need to concentrate on first since she uses white magic that will benefit her party, especially if you've felled one of her sisters. Therefore, concentrate your attacks on her for several rounds until she is slain. Have Cid, Cecil and Yang all physically attack - Tellah can simply defend for the duration of the battle. As for the other two sisters, simply lay into them like you did Cindy, and they will be no more. Their Delta Attack won't work once one sister is dead, and their magic otherwise is fairly weak. Just don't caste magic of your own on them, since they cast Reflect. The results of a powerful spell cast by you on the enemy party and then reflected back on you can be truly devastating.